The most obvious choice for the top spot, this free Keyword Research Tool has been a favorite for many years.
Not only is the Google Keyword Planner a good choice when starting a
new site, but it can also be your best friend as you grow and produce
more content.
The primary benefit of this tool is easy to see: you’re able to pull
information directly from Google, which is the search engine you’re most
likely to target, when optimizing your website.

Keywordtool.io
is what you should be using when you want to get tons of long tail
keywords for your seed keywords. This tool gives you keywords based on
Google, Bing, Yahoo and apple app store for free. It provides a huge
resource of keywords that would otherwise never see the daylight.

How it works:

Keywordtool
uses Google’s auto complete and ubersuggest’s letter adding features
(more on Ubersuggest in the next point) to pull out such huge keyword
variety. Adding alphabets to the beginning and end of your seed keywords
gives a natural keyword resource that can be used in your blog posts.

Ubersuggest
is a basic keyword research tool that gives you more than enough
keywords by simply appending letters to the end of your query. This
helps you to get access to so many long tail keywords that can be used
in your upcoming niche sites or even the main blog.

Ubersuggest has been useful if you have writer’s block too. You can use this tool to get lots of blog posts ideas that will boost your creativity.

How it works:

Ubersuggest
works on the principle of Google autosuggest. It appends letters
alphabetically to your seed keywords and then finds all the keyword
strings that come with the same letter set.

As noted above, there’s nothing better than getting advice straight
from the source. And, once again, Google is the source I’m talking
about.
Google Correlate is an often overlooked tool that can be extremely powerful for generating a large keyword list.
The primary reason to use this tool is the ability to see which
keywords get searched together. With this information, you can begin to
grow your keyword list (especially longtails).
In the example below, you’ll see that people who search for “diet plan” also search related phrases:

Ahrefs is one of the most popular Keyword Research Tool out there.
What I really like about them is extensive detail including the keyword difficulty. Ahrefs uses clickstream data
to also show how many clicks you will get from the search engine. This
is very useful after knowledge graph integration, as many keywords may
have huge traffic but they hardly get any clicks from search engine.
Reason being, they get answers directly from the Google search result.
Example of one such query is: “birthdate of any celebrity”

If you have any question about this topic, you can ask on comment box and forums